runcommand: do not replace %ROM% or %ROM_BN% tokens when empty

[twojstaryzdomu]

Empty %ROM% tokens will cause parameter issues for binaries that do not handle empty quoting without a parameter.

In some circumstances this makes sense - the binary has a sensible %ROM% default when empty.

Think empty %ROM% = game runs as default, %ROM% provided = user picked a custom level.

This PR addresses #3550.